Laser printers utilizing multiple lasers as light sources are known. Such laser printers are used primarily for one of two reasons as described below.
First, multiple lasers of the same wavelength are used to increase the printing speed of a laser printer by simultaneously scanning across and exposing a photosensitive medium with several laser beams. More specifically, these laser beams form several adjacent laser spots that are scanned simultaneously across a photosensitive medium during a sweep of a single polygon facet. Thus, several lines of the photosensitive medium are exposed simultaneously, enabling a faster laser printer.
Light intensity distribution of each laser spot at the photosensitive medium is approximately gaussian. The diameters of the exposed pixels are equal to the diameters of the laser spots at their 50% intensity level. One major problem with simultaneous, multiple spot printing is achieving sufficient overlap of the adjacent exposed pixels on the photosensitive medium to provide uniform exposed areas without image artifacts. Unless these pixels, and thus, the exposed scan lines have sufficient overlap of their light intensity profiles, the presence of individual scan lines on prints will be apparent and objectionable. Therefore, a printer that utilizes multiple lasers to simultaneously expose a photosensitive medium must have means for appropriate overlap of the exposed pixels and for producing appropriate spot sizes. The following patents describe different approaches for producing proper laser spot overlaps, and thus proper pixel exposure and proper scan line overlap at the photosensitive medium.
U.S. Pat. No. 4,253,102 discloses a printer that produces a desired scan line pitch (i.e., spacing between the scan lines) by utilizing an inclined semiconductor laser array having a plurality of laser light emitters. More specifically, these laser light emitters are arranged in a line that is tilted with respect to the line scan direction. In such arrays, all laser light emitters operate at the same wavelength. The pitch of the laser light emitters on this array is P.sub.o (as shown in FIG. 2 of this patent). Scanning across the photosensitive medium with the laser beams produced by the array that is tilted by an angle .theta. (See FIG. 3 of this patent ) results in the pitch of the laser spots at the photosensitive medium that is P'=P.sub.o cos(.theta.).
U.S. Pat. No. 4,393,387 also discloses a printer with a semiconductor laser array having a plurality of laser light emitters. This printer produces the desired pitch of the laser spots at the photosensitive medium, and thus the desired line pitch, by utilizing a prism that changes the apparent pitch of the laser light emitters. The pitch of the laser spots at the photosensitive medium in the cross scan direction can also be adjusted to a desired value by using reflectors as shown in U.S. Pat. No. 4,445,126.
Another method of adjusting the pitch of the laser spots is disclosed in U.S. Pat. No. 5,463,418 in which the centroids of the laser spot's intensity distributions are shifted closer to each other by using an aperture stop. This aperture stop is placed in the path of the laser beams and is located in front of a polygon. The frame of the aperture stop blocks off a portion of a laser beam's cross section, thereby creating non uniform laser spots and causing loss of light. U.S. Pat. No. 4,637,679 uses polarizing beam combiners to combine multiple laser light beams so they overlap in the primary scanning direction, but are separated by the required amount in the cross scan direction. Polarizing beam combiners absorb some of the light and thus cause loss of light.
It is also possible to write with more widely spaced scan lines as long as the scan lines in between are exposed in later scans. This method is called interleaving and is described in U.S. Pat. Nos. 4,806,951 and 4,900,130.
The above described laser printers are not color printers. They are not capable of producing color prints because all lasers operate at the same wavelength. In addition, in the above described laser printers, off-axis laser beams enter the post-polygon optics causing these laser printers to suffer from bowed scan lines. The problem of bowed scan lines is described later on in the specification.
A second reason for utilizing multiple lasers in printers is to print color images. This is done by exposing the photosensitive medium, which is sensitive to two or more wavelengths of light, by modulated laser beams of different wavelengths. This type of a laser printer is known and such printers are described in U.S. Pat. Nos. 4,728,965; 5,018,805; 5,471,236; 5,305,023; and 5,295,143. These laser printers are slow because they expose each pixel on the photosensitive medium with a laser beam of different wavelength and scan one line at a time.